In a fibre optical communications system it is desirable to be able to control the modulation depth of the light generated by the transmitting laser device. In order to maintain fast switching between states and reduce noise, the transmitting laser is not switched between some defined powers and then switched off, but its output is reduced to a low level. This modulation depth is also described as an extinction ratio (ER), the latter being the ratio of the optical intensity when there is a data ‘1’ and the intensity when there is a data ‘0’.
The current required by the laser to deliver these relatively high and relatively low optical outputs may not however be constant and may be affected by the tolerances between individual laser devices, and over time due to the variation of a single device's characteristics due to heating and/or ageing. Such variations can occur in normal operation as a device heats up in use. There are existing methods for controlling the modulation, but these are generally intended for systems that run continuously, and thus in their operation should allow for the settling time of a feedback loop.
However, in some communication systems, for example in passive optical network (PON) applications, it is common for a transmitter to operate in a burst mode, wherein the laser modulation levels should be under control after a very few pulses of a run-in sequence. In such cases there may be little time available for a control loop to settle.
Hence it is sometimes desired not only to be able to compensate automatically for manufacturing tolerances and parameter drift of the laser itself, but also to be able to reach a defined target modulation level within a very short time period, and with only a very small number of pulses being passed through the system.
Techniques for controlling the modulation (or ER) in a relatively steady state are known. Some are related to a technique presented by Smith (Electronics Letters, October 1978 pp 775-776), wherein a low amplitude low frequency (LF) modulation is added to the normal laser current. The fluctuations in the optical output from the laser at the known LF modulation frequency permits the estimation of the slope of the laser current/optical output characteristic, and hence the drive currents required for the desired average and modulated optical levels can be calculated provided that the laser characteristic does not have excessive non-linearity. It is thus possible to construct a feedback loop to maintain reasonable control over the modulation depth (or ER) provided that the feedback loop implied in this has time to settle.